1. Field of the Invention
The invention relates to a power control circuit which amplifies the modulated radio frequency (RF) signal according to the C class amplification of the analog mode or A class amplification of the digital mode. In particular, the present invention relates to automatic power control (APC) of the modulated RF signal.
2. Description of the Prior Art
FIG. 3 shows a block diagram of the conventional power amplifier having APC functions and operating as C class power amplifier. In FIG. 3, a modulated RF signal 1 is the signal to be amplified. A power amplifier 2 amplifies the RF signal. The gain of the power amplifier 2 is controlled by a signal applied to a gain control terminal 40. A signal 3 is the output signal from the power amplifier 2. A detector 4 detects the power level of the RF output signal 3. A comparator 5 generates a feedback signal by comparing the detected signal received from the detector 4 and the standard voltage. A level switching signal 6 is a predetermined voltage level which is provided for switching the standard voltage. A standard voltage generator 7 generates a standard voltage corresponding to the output level switching signal 6. A low pass filter 8 eliminates the influences of the output noises received from the comparator 5. A feedback signal 9 is an output signal from the low pass filter 8 in which the noises are eliminated by the low pass filter 8.
The operation of the above conventional art is explained hereinafter. In FIG. 3, the modulated RF signal 1 is inputted directly to the power amplifier 2 and is amplified up to a predetermined level and is outputted as a RF output signal 3. The RF output signal 3 must be within a predetermined level even if the RF input level 1 or the power source voltage happens to fluctuate.
Accordingly, it is necessary to observe the level of the RF output signal 3 and to feed back the detected signal to the amplifier 2 so that the fluctuation level is within a predetermined level. The level of the RF output signal 3 is detected in the detector 4 and the detected signal is inputted to the comparator 5. The detected signal is compared with the standard voltage in the comparator 5. The standard voltage is generated in the standard voltage generator 7 corresponding to the output level switching signal 6. The comparator 5 compares the detected level and the standard voltage and outputs the difference signal of the two. The difference signal passes through the low pass filter 8 where its influences of the noises is eliminated and it then becomes feedback signal 9. The feedback signal 9 is applied to the gain control terminal 40 to control the gain of the power amplifier 2. That is, if the power of the RF signal decreased, the feedback signal 9 operates so that the gain of the power amplifier will be increased, and if the power of the RF signal is increased, the feedback signal 9 operates so that the gain of the power amplifier 2 will be decreased. According to the above operations, the output power 3 of the power amplifier is maintained constant.
FIG. 4 shows a block diagram of the conventional power amplifier having automatic power control (APC) functions and operating as A class power amplifer. In FIG. 4, a variable attenuator 10 attenuates the modulated RF signal 1 in response to the signal applied to an attenuation control terminal 30. A bias switching device 11 supplies a bias voltage to a gain control terminal 40 of power amplifier 2 so that the power amplifier 2 operates at an optimum condition as an A class amplifer. The same reference numbers as used in FIG. 3 apply to the same portions or the corresponding portions of FIG. 4. Accordingly the detailed explanation of these portion is omitted in connection with the same reference numbers.
The operation of the above conventional art amplifier is explained hereinafter. In FIG. 4, the modulated RF signal 1 is inputted to the variable attenuator 10 and the attenuation factor is controlled by applying the feedback signal 9 to the attenuation control terminal 30. The attenuated RF signal is inputted to the power amplifier 2 and amplified in the power amplifier 2. The power amplifier 2 operates at an optimum gain condition as an A class amplifier by applying the bias voltage to the gain control terminal 40 from bias switching device 11. That is, if the power of the RF signal 3 is decreased, the feedback signal 9 operates so that the attenuation will be decreased, and if the power of the RF signal increased, the feedback signal 9 operates so that the attenuation will be increased. According to the above operation, the RF output power 3 of the power amplifier 2 is maintained constant.
As the conventional power control circuit is constructed as explained above, the characteristic of linearity is difficult to maintain in connection with the C class amplifier such as shown in FIG. 3. Also, the amplifier of FIG. 3 generates distortion if the amplifier is used as a digital amplifier such as for .pi./4 shift quadrature phase shift keying (.pi./4 shift QPSK). Further the amplifier of FIG. 4 shows low efficiency if the amplifier is used as an analog amplifier such as for frequency modulated (FM) signals having constant envelope, because the amplifier operates as a linear amplifier. Accordingly it is difficult to obtain a power amplifier which can be used for both analog mode and digital modes.
It is a primary object of the present invention to provide a single power amplifier which can be used for both analog mode and digital modes.
It is further object of the present invention to provide an effective C class operation in an analog mode, and to provide a low distortion linear characteristic for A class operation in the digital mode.